Manifold change good idea?

Cheers I'm going to do it. the only reason i dont have a bigger cam is the pistons have no valve relieves

Glad you're taking the advice to switch. A single plane intake "might" make more peak horsepower, but a dual plane will provide more improvement in the rpm range that the engine will operate in/spend time in most of the time. More "area under the curve", whether it be on a dyno chart or airflow chart, tends increase acceleration and overall performance even if the peak numbers are higher.

It wouldn't hurt to experiment with some jetting and accelerator pump squirter & cam changes to see if you can optimize the combo.